This invention relates generally to a process for depositing particles in a softenable layer to form a migration imaging member.
Migration imaging systems capable of producing high quality images of good density, continuous tone and high resolution are well known. Such imaging systems are disclosed, for example, in U.S. Pat. No. 4,084,966, the entire contact of which is hereby incorporated herein by reference. In a typical embodiment of these migration imaging systems, a latent image is formed on an imaging member comprising a substrate and a layer of softenable material containing electrically photosensitive migration imaging material. The latent image may be formed, for example, by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation, such as light. When the photosensitive migration imaging material is originally in the form of particles located just under the upper surface of the softenable material, the particles of the migration imaging material in the exposed areas of the migration member migrate toward the substrate when the member is developed by decreasing the resistance of the softenable layer sufficiently to allow migration of the migration imaging material in depth in the softenable material.
Various modes for developing such as softening the softenable layer sufficiently to allow migration of the migration material in depth in the softenable material are known. These various development modes include softening by liquid solvents, solvent vapors, heat and combinations thereof, as well as other methods of softening the softenable material to allow migration of the migration material in depth in the softenable material. Visualization of the latent image may also be affected by conventional dry or liquid electrostatographic toning techniques.
Various techniques may be utilized to deposit the migration imaging or marking material onto the surface of the softenable layer. These methods include coating a dispersion of particles in a volatile carrier onto the surface of the softenable layer and allowing the volatile carrier to evaporate; vacuum evaporating the migration material into the surface of the softenable layer; mixing marking particles with larger carrier particles and coating the surface of the softenable layer by cascading this mixture across the surface of the softenable layer as described, for example, in U.S. Pat. No. 2,618,551; coating the marking material onto the surface of the softenable layer by conventional coating techniques such as spraying, dipping, doctor blade coating, and draw down bar coating; and the like. The softenable layer is softened to permit embedding the migration material under the surface of the softenable layer.
One technique described in U.S. Pat. No. 3,598,644, the entire content incorporated herein by reference, involves vacuum deposition of migration marking material into the surface of a softenable layer by positioning a heat softened layer opposite a source of migration marking material vapors such as selenium. This technique produces particles of migration marking material in the surface of the softenable layer. However, the particles are undesirably small and do not scatter or absorb light sufficiently well to confer a high optical density on the film. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. Optical density is measured by diffuse densitometers with a blue Wratten No. 94 filter. When attempts are made to increase the size of the particles by increasing the mass per unit area of deposited material, some of the particles become so large that their lightscattering efficiency, for example to blue light, diminishes. Furthermore, the surface packing density of the particles may become low, with undesirably large gaps between the particles. Some of these gaps may be filled with very small particles, but these small particles do not scatter or absorb light very efficiently. The expression "surface packing density" is used herein to denote the ratio of the sum of the areas of the spheres projected normally onto the imaging menmber surface to the total surface area of the film. It is, therefore, not possible to obtain an optical density as high as could be obtained if all the particles could grow to about the optimum size for light-scattering with narrow size distribution and high surface packing density.
High optical density in migration imaging members allows high contrast densities in migration images made from the migration imaging members. High contrast density is highly desirable for most information storage systems. Contrast density is used herein to denote the difference between maximum and minimum optical density in a migration image. The maximum optical density value of an imaged migration imaging member is, of course, the same value as the optical density of an unimaged migration imaging member. The contrast densities in the order of 0.9 obtained with heat development of migration imaging members prepared with the vacuum deposition system described above were found to be undesirably low, for example, for preparing printing plates by contact exposure through imaged migration members. Thus, there is a continuing need for a better system for vacuum depositing larger migration imaging material particles with a narrow size distribution and high surface packing density to form a migration imaging member that is capable of forming images with high contrast density.